Currently, most wastewater treatment plants use an activated sludge process, based on biological oxidation of organic contaminants in a suspended growth medium. Oxygen is supplied from air using bubble type aerators. Efficiency of these systems is poor resulting in very high energy use. Tank size is large since oxygen demand loadings are low. The result is high capital and operating cost.
A second type of established biological oxidation process uses biofilms grown on a solid media. For example, the wastewater may be circulated to the top of the reactor and trickles down. Air is supplied at the bottom. The rate of oxygen transfer is limited by the biofilm surface area, and the operating cost is high because of wastewater pumping requirements.
Recently, development work has been done on a membrane supported bioreactor concept. For example, U.S. Pat. Nos. 4,181,604 and 4,746,435 describe a process for treating wastewater by supplying oxygen from one side of a gas-permeable membrane to micro-organisms growing on the other side of the membrane. Hollow fibers with porous walls were used as the membrane. In U.S. Pat. No. 5,116,506, a gas permeable membrane divides a reactor vessel into a liquid compartment and a gas compartment. A biofilm is grown on the gas permeable membrane on the liquid side of the membrane. Oxygen and alternate gases pass through the membrane to the bacteria growing on the liquid side of the membrane.